1. Field of the Invention
The present invention relates in general to producing acetaldehyde. More specifically, the present invention relates to producing acetaldehyde by hydrogenating acetic acid.
2. Description of the Related Art
Acetaldehyde is an important industrial chemical. It has been used as a starting material for the commercial manufacture of acetic acid, acetic anhydride, cellulose acetate, other acetate esters, vinyl acetate resins, synthetic pyridine derivatives, terephthalic acid, peracetic acid and pentaerythritol. Historically, acetaldehyde has been used to produce acetic acid, but improvements in technology have resulted in more economical acetic acid production from synthesis gas (a mixture of carbon monoxide and hydrogen). This development implies that it may be more economically attractive to produce acetaldehyde from acetic acid rather than to produce acetic acid from acetaldehyde if a technically viable route existed.
Acetaldehyde has been produced commercially by the reaction of ethanol with air at 480.degree. C. in the presence of a silver catalyst. This process has been replaced by the current process, the Wacker oxidation of ethylene. Both of these processes start with ethylene, and the Wacker route is more direct and efficient than the ethanol oxidation route. Acetaldehyde has also been produced by the hydration of acetylene. This process uses mercury salts as a catalyst, and mercury handling can cause environmental and safety problems. The use of acetylene causes safety concerns, and the high cost of acetylene relative to ethylene has rendered this process obsolete. Acetaldehyde can also be produced by reacting synthesis gas over a rhodium on silica catalyst at elevated temperature and pressure, but the selectivity to acetaldehyde is poor, and the process has never been practiced commercially. Acetaldehyde has also been produced from the reaction of methanol with synthesis gas at elevated temperature and pressure using a cobalt iodide catalyst with a group 15 promoter, but this process also has never been practiced commercially. Although the Wacker process is the preferred commercial process at this time, it also has many undesirable aspects. These include the special safety and handling problems associated with reacting ethylene with oxygen and the very corrosive nature of the aqueous acidic chloride-containing reaction mixtures which necessitates very expensive materials of construction. Thus a need exists for an acetaldehyde synthesis that is an improvement over the existing known processes.
A potentially attractive means to synthesize acetaldehyde is by the hydrogenation of acetic acid. See reaction (I) below. However the carboxylic acid group is generally considered to be among the most difficult functional groups to reduce by catalytic hydrogenation. Aldehyde groups, conversely, are easily reduced by catalytic hydrogenation to alcohols. See reaction (II) below. Thus, under the conditions required to reduce a carboxylic acid, the aldehyde is often not isolated in good yield because the aldehyde is further reduced to an alcohol. Furthermore, when the carboxylic acid contains an .alpha.-hydrogen, conversion to a ketone, water and carbon dioxide can occur. See reaction (III) below. This reaction becomes more prevalent as the number of .alpha.-hydrogens increases. Thus, acetone can be readily formed from acetic acid at the temperatures typically used for reaction (I) (300-400.degree. C.). The above discussed reactions related to hydrogen and acetic acid in the vapor phase are summarized below:
______________________________________ (I) CH.sub.3 CO.sub.2 H + H.sub.2 .fwdarw. CH.sub.3 CHO + H.sub.2 O .DELTA.G.sub.300.degree. C. = +0.8 kcal/mole .DELTA.G.sub.400.degree. C. = -0.04 kcal/mole (II) CH.sub.3 CHO + H.sub.2 .fwdarw. CH.sub.3 CH.sub.2 OH .DELTA.G.sub. 300.degree. C. = -0.4 kcal/mole .DELTA.G.sub.400.degree. C. = +2.5 kcal/mole (III) 2 CH.sub.3 CO.sub.2 H .fwdarw. CH.sub.3 COCH.sub.3 + CO.sub.2 + H.sub.2 O. ______________________________________
The hydrogenation of acetic acid to acetaldehyde and water (reaction (I)) is a mildly endothermic reaction. So, the thermodynamics of this reaction improve as the temperature is increased. The subsequent reaction (II), the hydrogenation of acetaldehyde to ethanol, is exothermic, and this reaction becomes less favorable as the temperature increases. Since the equilibrium of the acetic acid hydrogenation is poor, the reaction must be run with an excess of hydrogen to achieve appreciable acetic acid conversion. Thus, on a thermodynamic basis, ethanol formation will be favored at temperatures of 300-400.degree. C. Reaction (III), the formation of acetone, is essentially irreversible at all temperatures above 0.degree. C. and becomes very favorable thermodynamically as the temperature is increased. Increasing the temperature significantly above 400.degree. C. will not likely improve the selectivity to the desired acetaldehyde product because of increasing acetone production. Other reactions, such as the formation of methane, carbon oxides and C2 hydrocarbons also are relevant in acetic acid hydrogenation chemistry, but are of less importance than the three reactions described above unless excessively high temperatures are used. In some circumstances, the formation of ethyl acetate presumably through ethanol as an intermediate can also lower the selectivity to the desired acetaldehyde.
Thus, it appears that a major challenge in producing acetaldehyde via acetic acid hydrogenation is catalyst design. The ideal catalyst should facilitate the initial hydrogenation of acetic acid to acetaldehyde but have essentially no activity for the subsequent hydrogenation to ethanol nor for the dimerization reaction producing acetone. If a catalyst has even a small activity for conversion of acetaldehyde to ethanol or for the conversion of acetic acid into acetone, then extreme losses in acetaldehyde selectivity may occur if the reaction is operated beyond the equilibrium conversion level allowed for converting acetic acid and hydrogen into acetaldehyde and water. A need exists for a catalyst that selectively hydrogenates acetic acid to acetaldehyde.
Catalyst selectivity is only one requirement for a viable acetaldehyde synthesis. The synthesis must also be operated in a manner that will allow for the facile recovery of the very volatile acetaldehyde product, the recovery of byproducts and the recycle of unconverted reactants. Generally processes that hydrogenate carboxylic acids to aldehydes do so under conditions of about 1 bar pressure (all pressures given herein are in terms of absolute pressures) and hydrogen to carboxylic acid ratios approaching 50/1. Although these conditions may be sufficient for nonvolatile aldehydes, they are impractical for acetaldehyde which boils at 19-20.degree. C. Thus, a need also exists for a process that converts acetic acid into acetaldehyde in a manner that is selective and provides for the economical recovery of the acetaldehyde.
In spite of the thermodynamic limitations surrounding the hydrogenation of carboxylic acids to aldehydes, several examples of this reaction appear in the prior art. Generally these reactions are performed at about 1 bar pressure in the vapor phase in a large excess of hydrogen at temperatures ranging between 200 and 500.degree. C., and the reaction is most successful with aromatic carboxylic acids or aliphatic acids containing few .alpha.-hydrogens. Van Geem et al., in U.S. Pat. No. 5,336,810, describe a Mn/Zn/Al oxide catalyst that converts benzoic acid to benzaldehyde in the vapor phase at 330.degree. C. in a large excess of hydrogen in 88.3% selectivity at 98.9% conversion. Joentgen et al., in U.S. Pat. No. 5,059,716, describe catalyst system based on titanium or vanadium oxides in conjunction with one or more metals selected from Cr, Mo, Co, Ni, Zn, Cd and Cu for the hydrogenation of aromatic and aliphatic carboxylic acids containing not more than one .alpha.-hydrogen at 325-425.degree. C. at 1 bar in the presence of a large excess of hydrogen. Yokoyama et al., in Stud. In Surf. Sci. and Cat. 1994, 90, 47-58 and in Bull Chem. Soc. Jpn. 1993, 66, 3085-3090, describe the use of zirconium oxide and modified zirconium oxide catalysts for the hydrogenation of aromatic carboxylic acids to aldehydes under similar reaction conditions. Yokoyama et al., in U.S. Pat. No. 5,306,845, also describe the use of a purified chromium oxide catalyst for the hydrogenation of both aromatic and aliphatic carboxylic acids under similar reaction conditions. This patent gives several examples of the hydrogenation of high molecular weight acids, such as stearic acid. Acetic acid is also stated to be as a suitable acid, but no examples are given. Yokoyama et al. stress that the reason for the high purity requirement in the chromium oxide is to prevent the ketone formation reaction. Welguny et al., in European Patent Application EP 0 700 890 (1996), describe the use of oxide-supported tin catalysts for hydrogenation of a wide variety of carboxylic acids to aldehydes under the typical high-temperature, high-hydrogen, low-pressure conditions described previously. Although acetic acid is included in the claims of this patent application, the only examples are for aromatic carboxylic acids and for pivalic acid. Ferrero et al., in European Patent Application No. EP 539,274 (1993), describe Ru--Sn--B on alumina catalysts for hydrogenation of a wide variety of carboxylic acids to aldehydes under the typical high-temperature high-hydrogen low-pressure conditions described previously. Although the Ferrero patent application gives no examples for acetic acid hydrogenation, it is mentioned in the claims. Most of the Ferrero reference concerns the reduction of senecioic acid to prenal or the reduction of aromatic carboxylic acids to the corresponding aldehydes.
The most definitive work on the acetic acid hydrogenation to acetaldehyde is described by Ponec and coworkers in Recl. Trav. Chim. Pays-Bas 1994, 426-430, in J. Catal. 1994, 148, 261-269, in J. Molecular Catalysis A: Chemical 1995, 103, 175-180, in Applied Surface Science 1996, 103, 171-182, and in J. Catal. 1997, 168, 255-264. These workers have proposed a working mechanism for the reaction and have reported several examples of the conversion of acetic acid to acetaldehyde in good selectivity. The base catalysts for these reductions are partially reduced metal oxides having an intermediate metal-oxygen bond strength. Partially reduced iron oxide is the most selective metal oxide, and acetaldehyde selectivities almost as high as 80% could be obtained at 1.2 bar pressure and using a hydrogen/acetic acid ratio=50/1 at 321.degree. C. Addition of 5 wt. % Pt to this catalyst further increases the selectivity to acetaldehyde to over 80%. With tin oxide, the addition of the Pt about doubles the selectivity, increasing it from about 40% to about 80%. Ponec mentions in J. Catal. 1997, 168, 255-264 that there is an optimum Pt level, and that increasing the Pt level above 1.25 atomic % actually decreases the selectivity.
Although the acetic acid hydrogenation process studied by Ponec and coworkers is very selective to acetaldehyde, it is impractical as a commercial way to produce acetaldehyde. The impracticality stems from the need to isolate and collect acetaldehyde (normal boiling point=19-20.degree. C.) from a vapor stream where it is present in maximum concentrations of 2-3% (or less, depending on the conversion) at about 1 bar pressure. Water and byproducts must be removed from the mixture, and hydrogen and unconverted acetic acid must be recycled to the reactor. These operations require that the temperature be lowered considerably from the 300-400.degree. C. reaction temperature. A practical process requires much lower hydrogen/acetic acid ratios and much higher reaction pressures than used by Ponec.